1. Technical Field
The present invention relates to a displacement sensor with a confocal optical system that measures a displacement of an object in a noncontact manner.
2. Related Art
In the displacement sensor with the confocal optical system, a surface of a measurement object is illuminated with a laser beam, light reflected from the surface is received, and the displacement of the surface of the measurement object is measured based on a received light quantity level. At this point, the surface of the measurement object is illuminated with the laser beam whose coherent is kept at a high level, even if the laser beam passes through the optical system. Therefore, a measurement error is generated by a surface state of the measurement object. Specifically, when the surface of the measurement object is illuminated with the laser beam, pieces of light having different phases are reflected from the surface of the measurement object. The pieces of reflected light interfere with each other to be enhanced or cancelled, which affects the received light quantity. A spotted pattern corresponding to the surface state (degree of irregularity) of the measurement object is called a “speckle”.
The confocal optical system is utilized in the displacement sensor in which the laser beam is used. In the confocal optical system, because of a small spot size of the laser beam with which the surface state of the measurement object is illuminated, the high-contrast, large-size speckle is included in the light reflected from the surface of the measurement object. Accordingly, a speckle component included in a light receiving signal becomes a noise to increase the measurement error. In order to eliminate the measurement error, for example, Japanese Unexamined Patent Publication No. 2004-286598 discloses a displacement meter with the confocal optical system, in which displacement amounts in plural positions are continuously measured while a laser beam illumination position changes in the measurement object surface, thereby averaging the influence of the speckle.
In the displacement meter of Japanese Unexamined Patent Publication No. 2004-286598, although the measurement error caused by the influence of the speckle can be decreased, it is necessary that the displacement amounts in plural positions be continuously measured while the measuring point (laser beam spot position in measurement object surface) changes. Therefore, it takes a lot of time to perform the measurement.
In order to solve the problem, for example, Japanese Unexamined Patent Publication No. 2003-83723 discloses a three-dimensional shape measurement optical system in which the confocal optical system is used. In the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723, the measurement object is illuminated with a line beam while a lens position of the objective optical system changes instead of changing the laser beam spot position in the displacement meter of Japanese Unexamined Patent Publication No. 2004-286598. The light reflected from the measurement object surface is received by a line sensor, and three-dimensional shape measurement is performed based on a received light level. In the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723, it is not necessary to change the laser beam spot position in the measurement object surface. Therefore, the three-dimensional shape measurement optical system eliminates the problem of Japanese Unexamined Patent Publication No. 2004-286598 in that a long time is required to perform the measurement.
However, even in the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723, because the reflected light component includes speckle according to a surface state of the measurement object, the displacement is hardly measured with high accuracy. That is, on a short axis side of the substantially rectangular line beam with which the measurement object surface is illuminated, the speckle is generated in the reflected light like a focal distance of the spot of Japanese Unexamined Patent Publication No. 2004-286598. Because the line sensor receives the light on a long axis side, a variation in receive light quantity is generated in each pixel by the speckle. Accordingly, in the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723, a relatively wide area in the measurement object surface is illuminated with the substantially rectangular line beam. However, because the reflected light component incident to each pixel includes the speckle noise, a real detection value is hardly obtained even if displacement measured values corresponding to the pixels of the line sensor is averaged.
Additionally, in reading a signal from the line sensor, it is necessary to keep a lens position of an objective optical system constant in order to project the line beam onto the measurement object surface. Therefore, a high-speed response is hardly performed.
In the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723, it is necessary that a slit used to form a line beam and the line sensor be conjugatively disposed in order to obtain a confocal effect. Accordingly, because the precise positioning is required, the three-dimensional shape measurement optical system of Japanese Unexamined Patent Publication No. 2003-83723 is hardly assembled and adjusted.